The term “stent” is intended to indicate devices useful for endoluminal insertion (for example, in a blood vessel), usually effected by means of catheterisation, with subsequent deployment in place so as to achieve local support of the lumen. The primary purpose of the stent is to eliminate and avoid the restenosis (i.e., narrowing or closure) of the treated area.
For a general review of vascular stents, reference is made to “Textbook of Interventional Cardiology” edited by Eric J Topol, W. B. Saunders Company, 1994 and, in particular, to section IV of volume II, entitled “Coronary Stenting”.
Many patents also provide a general review of stents, for example, U.S. Pat. Nos. 4,503,569; 4,768,507; 4,776,337; 4,800,882; 4,830,003; 4,856,516; 4,886,062; 4,907,336; and EP 0 201 466A.
Notwithstanding the extensive research and experimentation in the stent field, as documented in the patent literature, only a relatively small number of operative solutions have, until now, found practical application. This is due to various factors, which include the following problems or requirements:
while moving toward the treatment site, the stent should be capable of adapting to the path, which may include various curved sections;
distortion of the stent while it is being positioned should not be detrimental to the ability of the stent to provide an effective support when it is positioned and deployed;
the longitudinal shortening effect which occurs in many stents upon deployment should be limited, if not avoided;
maximum homogeneity and uniformity in the expansion of the stent should be achieved at the desired location;
an extensive support surface should be provided to the wall of the lumen which is being supported;
the origination of complex shapes and/or possible stagnation sites, especially in blood vessels, should be avoided, in order to avoid undesirable phenomena such as coagulation or thrombosis; and
the stents should be able to be simply and reliably produced using available technology and they should incorporate the requirements listed above.
A stent is subject to various forces, including compression, flexure, and torsion. These stresses often cause the stent to perform in an undesirable manner. Additionally, a significant disadvantage of current stent designs is their failure to distribute these stresses throughout the structure of the stent. Each of these stresses is maximized in a particular area of the stent. In current stent designs two or more of these areas of maximum stress overlap. This results in at least two problems. First, an overlap of the maximum stress areas may overly fatigue the stent and cause failure in an area of overlapped maximum stress. Second, the failure to distribute or discharge the maximum stress of these forces at different areas causes stress concentration on the vessel wall which may cause vessel wall injury. Thus what is needed in the art is a stent meeting the requirements listed above that will avoid stress concentration and elastic distortion as well as provide good elastic matching between the stent and the vessel into which it is placed.